Field of the Invention and Related Art Statement
The present invention generally relates to a method for automatically adjusting a field of view of a television monitor system including a television camera and a display monitor, and more particularly to a door vision system having a television camera provided at an entrance of a house and a display monitor provided within the house, in which an image of a head of a guest who stands in front of an entrance door can be automatically displayed at a center of a display screen of the display monitor in a simple and less expensive manner.
In the door vision system generally called a television doorphone apparatus, a television camera is fixedly arranged on an outer wall of an entrance, so that the field of view of the television camera is also fixed. Upon arranging the television camera, an objective lens of the television camera is manually adjusted such that a head of a guest having an average height and standing at a usual position on a center line of the field of view of the television camera is displayed at a center of a monitor screen. However, a relative position of the head of the guest and the television camera is not fixedly determined. For instance, the height of the guests varies widely and further the guests might stand closer to or far away from the television camera or might stand on the right or left hand side with respect to the center line of the field of view of the television camera. Therefore, in the known television doorphone apparatus, it is not guaranteed that the head of the guest is always displayed at the center of the monitor screen.
In order to avoid the above mentioned problem, it is considered that the television camera or at least its objective lens is moved by a remote control such that the face of the guest is displayed at the center of the display screen. However, it is apparent that this solution is very expensive. Further, the operation of a user for moving the television camera or objective lens is very cumbersome.
In Japanese Patent Application No. 58-177,962, there is disclosed a known television system, in which a subject of an object displayed on a monitor screen is indicated manually by a user and then the subject is automatically traced in accordance with a brightness level of the subject. This solution is not a fully automated one and requires the manual operation of the user.
In Japanese Patent Application No. 60-289,873, there is also proposed another known television system, in which an image signal generated by a television camera is once stored in a frame memory and a difference between image signals of successive fields is derived. Then, the thus derived difference is processed to detect a contour of the head of the guest. However, this publication does not describe any concrete method or apparatus for finding the contour of the head.
It is considered that the image signal is processed by a central processing unit (CPU) in accordance with a program to find a position of the guest on the basis of pictorial characteristics and a part of the image is extracted from the picked-up image in accordance with the thus detected position of the guest. However, this solution has a drawback that the image processing operation for extracting the image of the head of the guest by using an algorithm for extracting the pictorial characteristics of the guest is very complicated and a very large amount of data has to be treated. Further, this solution requires high speed signal processing circuits and large scale memory.